Nucleosome-like complex of the histone from the hyperthermophile Methanopyrus kandleri (MkaH) with linear DNA

Growth-Phase-Specific Modulation of Cell Morphology and Gene Expression by an Archaeal Histone Protein

In all three domains of life, organisms use nonspecific DNA-binding proteins to compact and organize the genome as well as to regulate transcription on a global scale. Histone is the primary eukaryotic nucleoprotein, and its evolutionary roots can be traced to the archaea. However, not all archaea use this protein as the primary DNA-packaging component, raising questions regarding the role of histones in archaeal chromatin function. Here, quantitative phenotyping, transcriptomic, and proteomic assays were performed on deletion and overexpression mutants of the sole histone protein of the hypersaline-adapted haloarchaeal model organism Halobacterium salinarum. This protein is highly conserved among all sequenced haloarchaeal species and maintains hallmark residues required for eukaryotic histone functions. Surprisingly, despite this conservation at the sequence level, unlike in other archaea or eukaryotes, H. salinarum histone is required to regulate cell shape but is not necessary for survival. Genome-wide expression changes in histone deletion strains were global, significant but subtle in terms of fold change, bidirectional, and growth phase dependent. Mass spectrometric proteomic identification of proteins from chromatin enrichments yielded levels of histone and putative nucleoid-associated proteins similar to those of transcription factors, consistent with an open and transcriptionally active genome. Taken together, these data suggest that histone in H. salinarum plays a minor role in DNA compaction but important roles in growth-phase-dependent gene expression and regulation of cell shape. Histone function in haloarchaea more closely resembles a regulator of gene expression than a chromatin-organizing protein like canonical eukaryotic histone.

Histones comprise the major protein component of eukaryotic chromatin and are required for both genome packaging and global regulation of expression. The current paradigm maintains that archaea whose genes encode histone also use these proteins to package DNA. In contrast, here we demonstrate that the sole histone encoded in the genome of the salt-adapted archaeon Halobacterium salinarum is both unessential and unlikely to be involved in DNA compaction despite conservation of residues important for eukaryotic histones. Rather, H. salinarum histone is required for global regulation of gene expression and cell shape. These data are consistent with the hypothesis that H. salinarum histone, strongly conserved across all other known salt-adapted archaea, serves a novel role in gene regulation and cell shape maintenance. Given that archaea possess the ancestral form of eukaryotic histone, this study has important implications for understanding the evolution of histone function.

Genetic evidence for the importance of protein acetylation and protein deacetylation in the halophilic archaeon Haloferax volcanii

Protein acetylation and deacetylation reactions are involved in many regulatory processes in eukaryotes. Recently, it was found that similar processes occur in bacteria and archaea. Sequence analysis of the genome of the haloarchaeon Haloferax volcanii led to the identification of three putative protein acetyltransferases belonging to the Gcn5 family, Pat1, Pat2, and Elp3, and two deacetylases, Sir2 and HdaI. Intriguingly, the gene that encodes HdaI shares an operon with an archaeal histone homolog. We performed gene knockouts to determine whether the genes encoding these putative acetyltransferases and deacetylases are essential. A sir2 deletion mutant was able to grow normally, whereas an hdaI deletion mutant was nonviable. The latter is consistent with the finding that trichostatin A, a specific inhibitor of HdaI, inhibits cell growth in a concentration-dependent manner. We also showed that each of the acetyltransferases by itself is dispensable for growth but that deletion of both pat2 and elp3 could not be achieved. The corresponding genes are therefore "synthetic lethals," and the protein acetyltransferases probably have a common and essential substrate.

Asymmetry in the burial of hydrophobic residues along the histone chains of eukarya, archaea and a transcription factor

Background

The histone fold is a common structural motif of proteins involved in the chromatin packaging of DNA and in transcription regulation. This single chain fold is stabilized by either homo- or hetero-dimer formation in archaea and eukarya. X-ray structures at atomic resolution have shown the eukaryotic nucleosome core particle to consist of a central tetramer of two bound H3-H4 dimers flanked by two H2A-H2B dimers. The c-terminal region of the H3 histone fold involved in coupling the two eukaryotic dimers of the tetramer, through a four-fold helical bundle, had previously been shown to be a region of reduced burial of hydrophobic residues within the dimers, and thereby provide a rationale for the observed reduced stability of the H3-H4 dimer compared with that of the H2A-H2B dimer. Furthermore, comparison between eukaryal and archaeal histones had suggested that this asymmetry in the distribution of hydrophobic residues along the H3 histone chains could be due to selective evolution that enhanced the coupling between the eukaryotic dimers of the tetramer.

Results and discussion

The present work describes calculations utilizing the X-ray structures at atomic resolution of a hyperthermophile from Methanopyrus kandleri (HMk) and a eukaryotic transcription factor from Drosophila melanogaster (DRm), that are structurally homologous to the eukaryotic (H3-H4)₂ tetramer. The results for several other related structures are also described. Reduced burial of hydrophobic residues, at the homologous H3 c-terminal regions of these structures, is found to parallel the burial at the c-terminal regions of the H3 histones and is, thereby, expected to affect dimer stability and the processes involving histone structural rearrangement. Significantly different sequence homology between the two histones of the HMk doublet with other archaeal sequences is observed, and how this might have occurred during selection to enhance tetramer stability is described.

Genomes are covered with ubiquitous 11 bp periodic patterns, the "class A flexible patterns"

Background

The genomes of prokaryotes and lower eukaryotes display a very strong 11 bp periodic bias in the distribution of their nucleotides. This bias is present throughout a given genome, both in coding and non-coding sequences. Until now this bias remained of unknown origin.

Results

Using a technique for analysis of auto-correlations based on linear projection, we identified the sequences responsible for the bias. Prokaryotic and lower eukaryotic genomes are covered with ubiquitous patterns that we termed "class A flexible patterns". Each pattern is composed of up to ten conserved nucleotides or dinucleotides distributed into a discontinuous motif. Each occurrence spans a region up to 50 bp in length. They belong to what we named the "flexible pattern" type, in that there is some limited fluctuation in the distances between the nucleotides composing each occurrence of a given pattern. When taken together, these patterns cover up to half of the genome in the majority of prokaryotes. They generate the previously recognized 11 bp periodic bias.

Conclusion

Judging from the structure of the patterns, we suggest that they may define a dense network of protein interaction sites in chromosomes.

The two faces of Alba: the evolutionary connection between proteins participating in chromatin structure and RNA metabolism

The Alba superfamily of chromosomal proteins appear to have originated as RNA-binding proteins and to have been recruited to chromosomes possibly only within the crenarchaeal lineage.

Background

There is considerable heterogeneity in the phyletic patterns of major chromosomal DNA-binding proteins in archaea. Alba is a well-characterized chromosomal protein from the crenarchaeal genus Sulfolobus. While Alba has been detected in most archaea and some eukaryotic taxa, its exact functions in these taxa are not clear. Here we use comparative genomics and sequence profile analysis to predict potential alternative functions of the Alba proteins.

Results

Using sequence-profile searches, we were able to unify the Alba proteins with RNase P/MRP subunit Rpp20/Pop7, human RNase P subunit Rpp25, and the ciliate Mdp2 protein, which is implicated in macronuclear development. The Alba superfamily contains two eukaryote-specific families and one archaeal family. We present different lines of evidence to show that both eukaryotic families perform functions related to RNA metabolism. Several members of one of the eukaryotic families, typified by Mdp2, are combined in the same polypeptide with RNA-binding RGG repeats. We also investigated the relationships of the unified Alba superfamily within the ancient RNA-binding IF3-C fold, and show that it is most closely related to other RNA-binding members of this fold, such as the YhbY and IF3-C superfamilies. Based on phyletic patterns and the principle of phylogenetic bracketing, we predict that at least some of the archaeal members may also possess a role in RNA metabolism.

Conclusions

The Alba superfamily proteins appear to have originated as RNA-binding proteins which formed various ribonucleoprotein complexes, probably including RNase P. It was recruited as a chromosomal protein possibly only within the crenarchaeal lineage. The evolutionary connections reported here suggest how a diversity of functions based on a common biochemical basis emerged in proteins of the Alba superfamily.